276 CONFERENCE ON PALEOZOIC PALEOGEOGRAPHY 



death of the animal their shells are scattered far and wide, both on the 

 land and in the water. Many of ns in our field work have no doubt come 

 across small pools, sometimes a foot or less in diameter, swarming with 

 darting fresh-water ostracods or water fleas. In such instances, as evapo- 

 ration proceeds, the pool will become a fairly solid mass of ostracods, 

 and finally, when the water has disappeared entirely, their dead shells 

 will be scattered by the winds as dust, sometimes to considerable distances. 

 Fresh-water Ostracoda are therefore a factor in continental deposits. In 

 the sea a similar wide dispersal, independent of the animal's life history, 

 depends on the waves and currents, which bear the dead shells far from 

 their habitat in life and scatter them broadcast, so that their final resting 

 place may be in the deep-sea ooze or in the shallow littoral deposits. 



Most of the modern as well as ancient Ostracoda are of microscopic 

 size, and for this reason, even though in individual development they 

 probably exceed almost every other class, they must always remain an 

 inconspicuous element of any fauna. Another and more serious diffi- 

 culty, especially in the study of the fossil forms, lies in the simplicity of 

 shell structure found in some of the families. Among the recent fau- 

 nules, species and even genera are established on anatomical characters, 

 the shell being practically disregarded. It is a fact that several distinct 

 genera have shells with essentially the same outline and surface charac- 

 ters. The difficulty, if not impossibility, of distinguishing such genera 

 among fossil forms is obvious. For example, Bythocypris cylindrica, an 

 abundant fossil in practically all of the Middle and Upper Ordovician 

 formations, is closely differentiated from associated Cypridse, yet the name 

 possibly includes a number of distinct species. In outline and general 

 structure its shell can be duplicated in several genera of living forms. 

 On account of lack of character, this great group, which was more or less 

 abundantly developed from the Ordovician on to the present, will not be 

 mentioned in the further discussion. 



From the foregoing remarks the bearing of the Ostracoda on paleoge- 

 ography would seem to be insignificant had the class always possessed the 

 characteristics shown by many of the recent forms. However, judging 

 especially from their associates in the ancient continental seas, most of 

 the Paleozoic representatives were much more limited in their habitat. 

 Further, many of the Ordovician and Silurian species, particularly those 

 comprised in the family Leperditiidse, are not such inconspicuous fossils. 

 While the average recent ostracod seldom exceeds a millimeter in length, 

 certain Silurian Leperditias, the giants of the order, are 30 to 40 milli- 

 meters long. Again, there are hosts of forms like the Beyrichiidae of 



